Gli egiziani iniziarono una produzione seria del colore da circa il 4000 aC. Hanno introdotto il lavaggio dei pigmenti per aumentare la loro forza e purezza. Hanno anche introdotto nuovi materiali, il più famoso dei quali era il Blu Egizio – il primo blu prodotto intorno al 3000 a. C.

l pigmento è un silicato di calcio e rame (CaCuSi4O10) ottenuto mescolando una sale di calcio (carbonato, solfato o idrossido), un composto di rame (ossido o malachite) e sabbia (silice). Questo veniva riscaldato per produrre un vetro colorato ridotto in polvere per l'uso. Le vernici erano fatte utilizzando il pigmento di base con gomme o colla animale, che lo rendeva lavorabile e lo fissava alla superficie da decorare.

Può essere impiegato nell’affresco. Sconsigliato nella tempera, nell’olio e nell’encausto

Si può ottenere in modo relativamente semplice se la calce minerale (CaCO3), la sabbia (SiO2) e un minerale del rame (per es. malachite (Cu2(CO3)(OH)2) o azzurrite (Cu3(CO3)2(OH)2)) oppure del rame metallico vengono esposti all’ossigeno (O2) insieme con una piccola percentuale di un fondente come carbonato di potassio (K2CO3), sale (NaCl) o solfato di sodio(Na2SO4), vengono riscaldati a temperature tra 800 e 900 °C:

Nell'antico Egitto, molto spesso si usava come fondente il trona, una miscela di solfato di sodio, soda (Na2CO3) e cloruro di sodio, è stato utilizzato nella sintesi. La presenza di ossigeno (O2) dell’aria evita la formazione di rosso cuprite (Cu2O).

N.B. Le condizioni chimiche per la preparazione del Blu Egizio sono state trasmesse con estrema precisione. La prova che questo è stato fatto si trova nella straordinaria costanza della composizione chimica degli elementi Blu egizio in oggetti d'arte risalenti a più di 2500 anni,

Gli Egizi usavano anche la malachite, probabilmente il più vecchio pigmento verde conosciuto, e l’azzurrite, un pigmento blu verdastro. I due pigmenti sono chimicamente simili comprendendo di base un carbonato di rame (2CuCO3 • Cu(OH)2), e si trovano come minerali naturali che gli Egizi convertivano in pigmenti mediante frantumazione e lavaggio.

Sia l’azzurrite che la malachite appartengono alla classe dei carbonati di rame contenenti idrossido. I minerali di carbonato costituiscono una classe in cui gli anioni (CO3)2- sono legati da vari cationi nella cella elementare. In particolare I carbonati di rame sono carbonati minerali idrati che contengono sia il catione rame (Cu2+) che gli anioni ossidrilici (OH-). L’Azzurrite ha formula chimica Cu3(CO3)2(OH)2; la Malachite è Cu2(CO3)(OH)2.

Azurite: Cu3(CO3)2(OH)2 or Cu(OH)2 • 2(CuCO3) Malachite: Cu2CO3(OH)2 or Cu(OH)2 • (CuCO3)

Cella elementare Azzurrite

Cella elementare Malachite

I brillanti colori verdi e azzurri esposti da questi due minerali sono dovuti all'inclusione del rame, che è un agente pigmentante molto efficace, tra i costituenti chimici. Questo tipo di colorazione è chiamato idiocromatica. La Malachite possiede un rapporto di 1:1 Cu(OH)2 : CuCO3, mentre l’azzurrite possiede un rapporto di 1:2 per le stesse sostanze. Poichè Cu(OH)2 è più ossidato rispetto CuCO3 e la malachite possiede il più alto rapporto di questo catione, la malachite si ossida più facilmente dell’azzurrite. L'ossidazione più frequente della malachite è responsabile del suo colore verde brillante rispetto al blu profondo dell’azzurrite.

Quando sottoposta a lento riscaldamento, la malachite finemente triturata dà acqua e anidride carbonica (CO2), diventando alla fine ossido rameico nero (CuO). Si scioglie in soluzione acida e rilascia CO2, ma rimane di colore verde. Mentre, reagendo con NaOH caldo forma ossido rameico sulla superficie. Tuttavia il pigmento non reagisce con NaOH a freddo. Malchite in polvere sottile viene lentamente scurita anche dall’acido solfidrico (H2S).

Perugino, 1503, Museo Pinacoteca di S.Francesco, Montefalco, Italy

Perugino made a malachite green shirt on a green earth background.

Raphael, Madonna and Child Enthroned with Saints, The Metropolitan Museum of Art

The azurite blue of the Virgin's mantle has darkened due to its degradation into green malachite and now this mantle looks greenish.

La era la fonte dei minerali blu ottenute attraverso l'estrazione in epoca antica e medievale (lapis lazuli è chiamato in mineralogia lazurite e contiene quantità variabili di calcio (Na,Ca)8(AlSiO12)(S,SO4,Cl)). Nei tempi antichi, i lapislazzulo è stato valutato per la sua stabilità e per la sua brillantezza appariscente nei lapislazzuli purissimi.

Lapis lazuli was in ancient times mined in only one location, situated in the area of present-day Afghanistan (Badakhshan). It should be mentioned that in ancient China, the use of natural lapis lazuli was not as common as in other cultures (Persia, Mesopotamia, Egypt).

Due to its expense, ultramarine was often reserved for iconographically significant figures such as the Virgin Mary or Christ. To reduce the amount of pigment used, and thus the cost, ultramarine was often underpainted with carbon black, azurite, indigo, or green pigments such as green earth.

Lapis lazuli contains the mineral lazurite (Na8[Al6Si6O24]Sn), which is responsible for its bright blue color along with a variety of accessory minerals including: calcite, pyrite, amphibole, colorless pyroxene (often diopside), haüyne, sodalite, forsterite, muscovite, nosean, phlogopite, and wollastonite.

The intense blue color is due to the presence of the S3- radical anion in the crystal that partially replacing sulfate or chloride anions in cages within the tetrahedral aluminous-silicate framework. An electronic excitation of one electron from the highest doubly filled molecular orbital into the lowest singly occupied orbital results in a very intense absorption at λmax~617 nm.

The almost ubiquitous, but unstable azurite is a mineral containing copper (Cu3(CO3)2(OH)2). Depending on its environment, it will eventually transform into malachite, a green pigment, and is unsuitable for outdoor use.

In the traditional method of producing pigment from lapis lazuli, as outlined by Cennini, these minerals are removed by a lengthy purification process which, when repeated several times, results in several grades of pigment, each less saturated in color than the one before. The last, crudest, grade is typically referred to as ultramarine ash. Calcite generally remains present, even following extraction by the Cennini method

Lazurite is easily identified by Raman spectroscopy by the presence of a strong band centered near 549 cm-1 (the S3-

symmetric stretching mode) and a weaker band centered between 582 and 586 cm-1 (the S2- symmetric stretching mode).

Raman spectrum acquired from a 14th century Italian illuminated manuscript, from the Laudario of Sant’Agnese by Pacino di Bonaguida; 785 nm excitation, 1.25 mW/μm2, 10 s acquisition. The starred peak is the main diagnostic feature for ultramarine pigments, and the band at 1049 cm-1 is likely due to the presence of the pigment lead white (2PbCO3 · Pb(OH)2) mixed with the ultramarine.

Lapis lazuli (the mineral lazurite (Na,Ca)8(AlSiO12)(S,SO4,Cl)) was in ancient times the only stable and durable mineral blue. Azurite, which is unstable, was often used as a chemical substitute. Based on findings of some groups, it is very likely that Ultramarine Blue (typical formulation Na6.9[Al5.6Si6.4O24]S2.0), which is chemically very closely related to the mineral lazurite, was artificially produced in ancient China.

In the early 10th century AD, the blue mineral vivianite (Fe3(PO4)2•8H2O) became a speciality of the European medieval times, mined in areas north of the Alps, and was used as a pigment.

is an unstable iron phosphate mineral with variable Fe(II/III) content that is completely colourless if kept in an oxygen-free environment (solely contains iron(II)) and that, if exposed to oxygen, will over time oxidise into blue and eventually brown compounds with a higher iron(III) content.

Vivianite owes its diagnostic indigo-blue color to intervalence charge transfer (IVCT) between iron ions filling dimers of edge-sharing octahedra in the crystal structure. IVCT indicates: the excitation of an electron from a cation of low oxidation state and subsequent transfer to a neighboring higher oxidation state cation. It is often induced by visible light with a suitable wavelength and produces a characteristic color. For the process to occur, cation electron orbitals should overlap so the electrons can hop back and forth.

However, the aging of blue pigments based on vivianite in old paintings has lately become a serious concern. The mineral oxidation product of vivianite is the brown santabarbaraite pseudomorphus, in which half of the iron atoms are iron(II) and the other half iron(III).

Cornelisz van Poelenburgh (1586-1667), Adoration of the Shepherds; (Fig. 1, left).

The robe of Virgin Mary is bleached by an alteration of vivianite and ultramarine pigments. The cross section (Fig. 2, right) shows the paintning`s stratigraphy in the affected area (two underlayers of altered, changed vivianite and an upper blue ultramarine layer).

in oil paint a color loss due to a Fe2+→Fe3+ oxidation is also possible without destabilization of the vivianite structure.

The area of ancient China in which, according to the current state of knowledge, the synthetic pigments Chinese Blue and Chinese Purple, also called Han Blue and Han Purple, were produced; the area extends on a relatively limited territory about 200–300 km north of the ancient city of Xian. Today it is thought that, in this area in northern China, Ultramarine Blue (‘‘artificial lapis lazuli’’) might also have been produced (y≈800 BC)

Han Blue and Purple are compounds based on copper silicates, as well. The production of Han Blue (BaCuSi4O10) and Han Purple (BaCuSi2O6) is generally more difficult than the production of Egyptian Blue.

In a first step, a barium mineral (generally barite (BaSO4) or witherite (BaCO3)) was exposed for several hours to quartz (SiO2), a copper mineral and an essential lead salt supplement at a temperature of 900–1000 °C.

For the production of larger amounts of Han Blue, these conditions are not applicable as shown in the following typical equation, the temperature will be higher by approx. 100 °C.

The limited availability and the high stability of the rare barium minerals had a restrictive effect on these syntheses. Thus, it was necessary to reach relatively high temperatures for the synthesis.

Lead salt additives serve a chemical double function: on the one hand they assist the catalytic decomposition of barium minerals at lower temperatures and on the other they serve as fluxes in a similar way to the additives in the preparatives of Egyptian Blue.

The purple shade of Han Purple comes from the red impurity of copper(I) oxide, Cu2O, (mineral name cuprite), that is slowly generated by the decomposition of Han Purple, which probably happens as outlined in the following chemical equation.

At temperatures of more than 1050 °C, this decomposition takes place at a quite fast rate. The production of copper(I) oxide depends on the conditions of preparation but, based on the traced ancient synthetic procedures, this problem could not be avoided in BaCuSi2O6 production in ancient China.

Han Purple is very unstable from a chemical point of view. While the copper(I) oxide in Han Purple stayed stable and a decomposition of Han Purple progressed, the purplish colour of the artefacts increased.

Pure BaCuSi2O6 (Chinese ‘‘Purple’’) (a), to which cuprite (Cu2O) (g) was gradually added. The samples (b–f) contain increasing amounts of cuprite. According to the increasing amount of cuprite, the shade becomes more reddish. ‘‘Normal’’ Han Purple, which is produced in a synthesis at approx. 1000 °C, comparable to the syntheses conducted in ancient times, is, in terms of the colour shade, similar to sample (c).

The two chemically very closely related compositions CaCuSi4O10 (Egyptian Blue) and BaCuSi4O10 (Han Blue) differ only in the way they exchange the earth alkali element. The two compounds have the same basic structure and very similar properties. They both have layered structures with (SiO)4 silicate squares forming the structural framework. Four of those (SiO)4 four-ring units form new (SiO)8 four-ring units through condensation and new connections. Opposite four-ring units become so close that their terminal Si–O- groups take up a copper ion in a square planar arrangement.

These copper ions are the colouring agent (chromophore). They are very tightly bound in the stable silicate matrix and cannot be removed easily by chemical and physical means. This tight binding is the key to the high stability of Egyptian and Han Blue. Heat, strong acids and light cannot harm these two pigments.

The chromophore copper is not a very efficient colouring agent, however; consequently, it needs quite a fair amount of material to reach a certain intensity in colour. The layered structure leads to the formation of platelet crystals that feature anisotropic interaction with light. The crystals show dichroism, which can be seen in the substance’s lighter appearance when grinding is applied. Different shades of darker or lighter blue can thus be produced

Han Purple features a layered structure, as well, but its framework differs greatly from the structure of Han Blue. Its basic units are isolated (SiO)4 four-ring units, whose terminal oxygen atoms bind two connected copper atoms. This results in the formation of an infinite arrangement of Cu2 units

Owing to its Cu–Cu bond structure, Han Purple has a low chemical stability. Even weak acids wear and bleach it; hence, a light-blue mixture of Ba/Cu oxalate is formed under the influence of oxalic acid that may occur under natural circumstances, for instance by excretion of certain micro-organisms and lead to destruction of pigment layers of paintings containing Han Purple.

Artificial lapis lazuli: Ultramarine Blue.

Lapis lazuli (the mineral lazurite

(Na,Ca)8(AlSiO12)(S,SO4,Cl) in altri termini è un calcare mineralizzato contenente dei cristalli cubici di lazurite)

was in ancient times the only stable

and durable mineral blue. Azurite,

which is unstable, was often used as a

chemical substitute. Based on findings

of our group, it is very likely that

Ultramarine Blue (typical formulation

Na6.9[Al5.6Si6.4O24]S2.0), which is

chemically very closely related to the

mineral lazurite, was artificially produced in ancient China.

Nowadays, Ultramarine Blue can be very easily obtained in the presence of sodium salts (sodium carbonate), sulfur compounds and in alkaline conditions compounds and in alkaline conditions.

The blue colouring of sulfur radical ions was probably produced by the reduction of the existing sulfate with spectroscopically detected carbon particles, which were contained in the plant ashes (basically potash K2CO3 and soda Na2CO3). Ultramarine Blue is generated at relatively low temperatures (400–600 °C); this has been done since the early 19th century, when the industrial production process first came into use.

The chromophore of Ultramarine Blue and lapis lazuli is the blue S3-2 radical ion

incorporated in sodalite cage structures ([Al6Si6O24]-6). This naturally very unstable ion becomes very stable in its incorporated form by chemical protection in a solid matrix. Due to its chemical similarity, Ultramarine Blue is also denominated

‘‘artificial lapis lazuli’’. The surrounding matrix does not significantly influence

the essential colour properties of the S3- ion.

Ultramarine Blue and lapis lazuli feature a significantly higher degree of light absorption than copper silicate pigments. The blue tone is very intense, even if prevalent only in small amounts in pigment mixtures. However, it must be highlighted that the yellow S2

- radical is also unavoidably incorporated during any kind of preparation, as is the case with natural lapis lazuli. A higher ratio of S2

- radicals ‘‘dilutes’’ and lessens the blue pigment properties or may, in still higher concentrations, lead to a greenish tone due to the mixture of blue and yellow or even to a green pigment that previously has not been used. The rare natural green tone of mineral lapis lazuli leads to a considerable enhancement in value.

Maya Blue was developed by the Indian

cultures of Central America on a very

different chemical basis than the

aforementioned pigments. It is derived from

the colour indigo, which was already

known in ancient times. By means

of a ‘‘high-tech’’ process, the Indians

embedded indigo in white clays

(palygorskite ((Mg,Al)4Si8(O,OH,H2O)24) or

sepiolite), for which temperatures between

150 and 200 °C were necessary.

Indigo

The pigment was made by burning incense made from tree resin and using the heat to cook a mixture of indigo plants and a type of clay called palygorskite. As the mixture was heated, indigo molecules filled a network of tiny channels inside the clay. Some of these bits of indigo plugged the pores on the surface, preventing the color from escaping over time. The clay, in turn, protects the indigo from the environment. Harsh chemicals can destroy a sensitive bond within indigo molecules -- changing the color from blue to yellow. Like the double-parked car that prevents you from opening the driver-side door to your own car, the clay channels take up the space around the bond, blocking these chemicals.

Modular structure of Palygorskite (left) and Sepiolite (right) viewed along the c-axis. H2O in the channels is not shown.

In tube-like channels of palygorskite or sepiolite clays, indigo molecules are intercalated. The guest molecule indigo is the chromophore. It has long been argued whether the indigo molecules are located in the core of the clay or whether they are adsorbed tightly at the clay’s surface. Thorough investigations have shown that the indigo molecules are clearly incorporated in the clay.

The channels which take these molecules up provide much more efficient chemical protection than residing on the surface. In contrast to ‘‘free’’ indigo, intercalated indigo thus does not fade, even under harsh conditions. For instance, the photochemical properties of the guest indigo had improved to such an extent that Maya Blue became a valuable pigment suitable for outdoor use. Some of those stable indigo clays are even more brilliant in appearance than natural indigo, which is considered to be due to the presence of iron nanoparticles formed during processing from the indigo plant raw materials.

Thee fading of indigo paint is caused by the chemical breakdown of the component: indigotin. The extent to which this process takes place depends on a variety of factors.

Frans Hals, St. George civic guard 1627, Frans Hals Museum. The indigo has faded to a greyish light blue.

Frans Hals, St. Adrian civic guard, 1627, Frans Hals Museum. The indigo blue colour of the sashes has been well preserved.